Apparatus for thermal decomposition of metal halides



May 1, 1951 c. scHl-:ER ETAL 2,551,341

APPARATUS FOR THERMAL DECOMPOSITION 0F METAL HALIDES Filed Nov. 22, 1949 2 Sheecs-Sheei'l l Patented May l, 17951 APPARATUS FOR THERMAL DECOBIPOSL.-

TION QF METAL EALI'DES Charles L. Scheer, Larchmont, and Charles M. McFarland, Lehighton, Pa., assignors to The New Jersey Zinc Company, New York, N. Y., a

corporation of New Jersey Application-November 22, 1949, Serial No. 128,756

6 Claims. 1

This invention relates to the production of metals and, more particularly, to apparatus for producing metals by thermal decomposition of a metal halide. The apparatus of our invention is particularly adapted for the production of such metals as titanium, zirconium and hafnium by thermal decomposition of a halide of such a metal. ,f

Metallic titanium of the highest purity is produced at present by the thermal decomposition of a titanium halide such as titanium tetraiodide. The apparatus used for this purpose comprises a chamber capable of being evacuated so that an atmosphere ci the titanium halide can be established therein substantially free of oxygen, nitrogen, and the like. The halide-decomposition surface comprises a lament of tungsten or titanium heated by the passage of electric current therethrough. At the start of operation, the fine wire filament is heated by a relatively small current at moderate voltage. As metallic titanium is deposited on the heated wire by decomposition of the titanium halide in contact therewith, the cross-sectional area of the filament increases. Withthe resulting increase in electrical conductivity of the filament an increasingly greater current, at progressively decreasing voltage, is required to maintain the lament at the halidedecomposition temperature. Operation of the apparatus is thus limited by the electric current requirements of the enlarged filament. Such apparatus also requires electrical equipment capable of supplying the requisite lament heating power over an exhorbitant range of inversely varying voltage and current. The relatively small contact surface provided by the heated filament also imposes a limit on the rate of production of the metallic titanium. Accordingly, there is a demand for apparatus capable of eifecting thermal decomposition of a titanium halide without the unreasonable electrical requirements and non-commercial limitations of the filament-type devices now in use.

We have devised an apparatus for producing metallic titanium by thermal decomposition of atitanium halide which is more moderate in its electrical heating demands and which appears to be adapted to commercial-scale output of high-purity metal. closed vessel provided with hermetically sealed walls and adapted to be evacuated, a titaniumdeposition shell positioned within said vessel in such manner as to dene a chamber between the shell and the walls'of said vessel, an electrical heating element positioned within the shell and Oui' apparatus comprises a` adapted to heat the shell essentially by thermal radiation to a temperature suicient to effect thermal decomposition of the titanium halide in contact therewith, means for evacuating the shell and the chamber, and means for establishing a titanium halide atmosphere in said chamber substantially out of contact with the heating element. Metallic titanium is deposited on the surface of the heated shell in contact with the titanium halide atmosphere. Inasmuch as the shell is heated essentially by radiant heat from the heating element, and not by any current which may pass through the shell itself as part of the heating element circuit, the electrical resistance of the heating element is not altered by the building up of a metallic titanium deposit on the shell surface.

The apparatus of our invention will be more fully understood by reference to the accompanying draw-ings in which Fig. 1 is a front elevation in cross section showing general configuration and details in the construction of our apparatus;

Fig. 2 is a partial view in cross section of a modification of our apparatus; and

Fig. 3 is a partial View in cross section of a modified form of our apparatus particularly useful in producing pure titanium from the relatively impure metal.

As shown in Fig. 1, our apparatus for the production of metallic titanium comprises a closed vessel 5. The walls of the vessel are .hermetically sealed so that the vessel may be evacuated. Thus, the vessel may comprise a vertically disposed cylinder 6 provided at its lower end with a closure plate l welded thereto and closed at its upper end by a removable cover member 8. The joint between the cover member and the end of the cylinder may be hermetically sealed, even at elevated temperatures, by means of a metallic gasket such as an aluminum ring 9, the cover being clamped tightly thereagainst by lock bolts I0. This sealing arrangement is described in the application of one of us, Charles L. Scheer, Serial No. 101,853, filed June 28, 1949, for Sealing Structure of Gasket Type.

Within the closed vessel 5 there is positioned a titanium-deposition shell II. The shell is advantageously constructed in the form of a substantially closed cylinder I2 which provides a titanium-deposition surface I3 Within the chamber I4 defined by the shell II and the vessel 5. The lower end of the shell may be appropriately closed by a plug I5 or the like in threaded engagement with the cylinder. The plug is advanthreaded into a collar l1 mounted on the innerV The shell is constructed surface of Ythe cover. of material Such as graphite, molybdenum, or titanium capable of withstanding continuous operation at temperatures of at least about 1200Q C. without deterioration and Without `appreciably.

contaminating the deposited metallic titanium.

The shell ll is adapted to form a protective casing about an electrical heating element I. The heating element should {be lconstructed of appropriate material capable of operatinga-t a' temperature sulicient to heat the titaniumdeposition shell H to a temperature olf at least about 1200 C. For this purpose, a spiral graphite resistor such as that shown in Fig. l has been found to be particularlyV satisfactory provided that 'a substantially non-oxidizing atmosphere is maintained within the shell. The spiral graphite resistor can be formed readily from a solid rod by drilling a hole axially thereinto and by then cutting a spiral groove in at least a portion of the resulting sleeve-like structure. Other suitable materials may be used in lieu of the graphite resistor, however. For example, platinum, advantageously in ribbon form and wound upon a refractory core, may be used. The lower end of the heating element is advantageously supported by contact with the walls of a well formed in the end plug I5 of the shell. The upperend of the heating element is supported by a head assembly 2| advantageously constructed as described in the copending application of one of us, Charles L. Scheer, Serial No. 101,854, filed June 28, 1949, for Gasket Type Sealing Structure.

The heating element head assembly is mounted at the upper end of a tubular well 22 the lower end of which communicates with an opening 23 in the removable cover 8. The head assembly provides a vacuum-tight seal as well as electrical insulation between the tubular well 22 and a heating element terminal post 24. The lower end of the terminal post 24, positioned within the well 2G, is advantageously threaded into a metal connector 25 which, in turn, is connected, by threads or otherwise, to the otherwise unsupported upper end of the heating element. The electrical circuit for the heating element 'thus comprises the terminal post 24, the metal con;

nector'25, the heating element I8, the end plug l5 and the shell cylinder I2 which is connected to the removable cover 8 of the outer closed vessel. The heating element connections are thus made between the upper end 2t of the terminal post 24 and a shell terminal post 2l'.

t will be appreciated from the foregoing discussion that the electrical resistance of the heating element is not materially altered by deposition of metallic titanium substantiallyv only on the outer surface of the shell cylinder i2. However, the electric circuit including the shell cylinder and the heating element is susceptibleA to design such that the heat supplied to the shell cylinder is automatically increased to compensate for the increasing difficulty in maintaining the shell surface at the desired halide-decomposition temperature. Thus, if the shell cylinder is characterized by an initial electrical resistance which is an appreciable portion of the resistance of the heating element circuit, a decrease in the resistance of the shell cylinder 4 will result in an increase in the current flow-4 ing through the circuit with a corresponding increase in the amount of heat emitted by the heating element. The required decrease in the electrical resistance of the shell cylinder is provided by ithe ldeposition v of metallic titanium thereon with resulting effective short-circuiting of the shell cylinder as a 'component of the circuit. For example, if the cross-sectional dimension of the shell cylinder be so chosen, with respect to .the heating element resistance, that the shell cylinder resistance comprises about 10% of the vtotal circuit resistance, an effective elimination of the .shell cylinder resistance by ldeposition of titanium thereon will result in an increase of about 10% in the heating element current. A s a result, the increase in heat emitted by the heating element, minus the heating lost by the elimination of the shell cylinder as a supplemental heater, will amount to a net increase of about 10% measured Vat the shell surface. It will be clearly apparent that by appropriate design of the shell cylinder and heating element, effective automatic control of the heat supplied to the deposition surface .of the shell cylinder can be achieved.

Evacuation of the apparatus is eiected through a vacuum line 28 communicating with the upper portion of the closed vessel. Evacuation is relied upon not merely as a means of substituting a titanium halide atmosphere for the normal atmosphere within'the chamber i4 but also to completely degas the apparatus so that occluded gases such as oxygen, nitrogen, and the like, will not be liberated from the elements of the apparatus during the titanium-deposition operation with resulting contamination of the deposited metal. Accordingly, the heating element is operated while the apparatus is being evacuated so that the entire device will be raised to its normal operating temperature and therebyl effect degasing of its interior prior to the admission thereto of the titanium halide. Gases within the shell are withdrawn therefrom through the small opening I6 in the shell plug l5 so that both the shell Il and the chamber I4 may be simultaneously evacuated. Alternatively, plug I5 may not be provided with the aforesaid opening vand in lieu thereof the vacuum line may communicate with both the chamber I4 and the well 20 of the heating element head assembly through a T-connection 30, as shown in Fig. 2. However, if the shell cylinder is constructed of graphite or the like, which is relatively pervious to the passage of gas there4 through, neither the opening i6 nor the T-connection 30 need be provided. In such a case, the gases may be completely removed from the entire apparatus simply through the single Vacuum line 28. The titanium halide is advantageously admitted adjacent the opposite or lower end of the apparatus through a charge line 3l only after a high vacuum has been attained within the apparatus at normal operating temperature.

In operation of the apparatus, the cover 8, carrying the titanium-deposition shell and heating element assembly, is clamped in place so as to hermetically seal the main outer Vessel 5 and the vacuum line 28 is connected to a vacuumv pump. When a partial vacuum has lbeen obtainedv within the vessel, the heating element is placed inoperation so as to raise both` the element,` the surrounding shell, andthe interior of the vessel 5 to their normal operating temperature. The vacuum pumping is continueduntil an absolute presa the sure f about '1/2 micron is attained within the vessel, the development of such a vacuum within the vessel being indicative of effective degasing of all elements therewithin. The vacuum line is then closed so that the pump can be disconnected orv stopped, and the apparatus is ready for the production of metallic titanium.

The charge of titanium halide, such as titanium tetraiodide, is advantageously introduced in the vapor form through the charging line 3i. The titanium halide vapors iill the chamber between the shell I I and the outer vessel and also penetrate, in the apparatus shown in Fig. l, into the interior of the shell II through the plug opening I. The titanium halide Vapor coming into contact with the outer surface of the shell II, which is maintained at a temperature of at least about l200 C., decomposes and deposits metallic titanium thereon. The relatively small amount of titanium halide which enters the shell II is similarly decomposed with the liberation of free iodine vapor. The small amount of titanium metal thus produced Within the shell in the vicinity of the heating element has virtually no effect upon the current-carrying' capacity of the heating element. The formation of iodine vapor within the shell Il, resulting from a decomposition reaction which produces more than 1 molecule of iodine for each molecule of titanium tetraiodide which is decomposed, provides a relatively inert atmosphere Within the shell and effectively excludes the admission thereof of appreciable further quantities of the titanium halide. Thus, in the apparatus shown in Fig. l, the heating element is substantially out of communicating contact with the titanium halide maintained in the chamber I4. In the form of apparatus shown in Fig. 2, wherein separate vacuum lines communicate with the shell EI and chamber I4, with the shell plug impervious to the flow of thephalide therebetween, no halide will be admitted tothe interior of the shell at any time.

Iodine vapor produced in the decomposition chamber Id, and accompanied by vapors of titanium iodides, is permitted to diuse out of the chamber through the vacuum line 28. By providing a condenser about the vacuum line, the iodine and titanium iodide vapors may be selectively condensed so as to prevent the development of static charging conditions within the chamber and to permit recirculation of the titanium iodides. Accordingly, the titanium halide vapors may be continuously introduced into the chamber so as to continuously maintain an essentially titanium halide atmosphere for contact With the titanium-deposition surface I3 of the heated shell. Operation of the apparatus can thus be continued for a sufficient period of time to build up on the outer surface of the shell a relatively thick deposit of metallic titanium. The thickness of the deposit can be observed, if. desired, through a sight tube 32 provided in the cover B. When a suiicient deposit of metallic titanium is formed on the outer surface of the shell II, the titanium halide charging line is closed, the heating element is turned off, and the apparatus is allowed to cool to a temperature at which it is safe to open the vessel without danger of the deposited titanium being oxidized or otherwise contaminated by contact with the incoming airl The ductile titanium deposited on the surface I3 of the shell can be readily removed therefrom, for example, by cutting through the deposit longitudinally of the shell and by then peeling or.

bending the depositfoutwardly" awayfromthe shell surface. We have found that the deposited titanium is of substantially that degree of purity characteristic of the metal produced by the halide-decomposition processes heretofore in use. There is only a trace of titanium carbide formed on that portion of the titanium deposit in contact with the graphite shell, and, if desired, this relatively thin ilm of the carbide can be removed by mechanical means. The apparatus of our invention is not limited to the use of titanium tetraiodide as the charging material which is thermally decomposed to produce metallic titanium. By appropriate choice of operating temperature, both titanium tetraiodide and titanium tetrabromide may be used as the titanium halide forming the atmosphere within the chamber I4, and there appears to be no reason to doubt the utility of the apparatus in handling other halides capable of thermal decomposition. The titanium halide may be charged as such through the charging line 3|, or the halide may be produced in situ within the decomposition chamber. For example, a foraminous grid 33 may be positioned Within the decomposition chamber in a position spaced from both the shell II and the walls of the vessel 5, as shown in Fig. 3. The space between the grid and the side Walls of the outer vessel is then lled with spongeV titanium produced by any conventional reduction method, the sponge metal generally being characterized by a degree of impurity inferior to that obtainable by thermal decomposition of a titanium halide. The desired titanium halide can then be produced in situ Within the chamber by introducing the corresponding halogen vapor, such as iodine or bromine, through the charging line 3l. The sponge titanium, being maintained out of contact with the surface of the heated shell II, is nevertheless raised by heat radiated from the shell to a temperature at which the impure titanium will react with the halogen vapor. The resulting titanium halide vapor diffuses through the grid into the remainder of the chamber I4 and is available for decomposition on the hot deposition surface I3 of the shell as previously described.

It will be seen, accordingly, that the apparatus of our invention is characterized by the maintenance of the heating element in a controlled non-oxidizing atmosphere, substantially or even completely out of contact with the titanium halide vapors. By maintaining the heating element in such a non-oxidizing atmosphere (i. e. either in an atmosphere consisting essentially of iodine or in a substantial vacuum), the heating element may be operated at a suciently high temperature to heat the titanium-deposition shell solely by radiant heating to a titanium halide-decomposition temperature. Inasmuch as the heating element is not exposed to an ever present supply of the titanium halide, the electrical conductivity of the heating element is not subject to alteration during the titanium deposition operation. Deposition of metallic titanium takes place substantially only on the surface of the shell surrounding the heating element. Moreover, the relatively large decomposition surface of the shell surrounding the heating element lends itself .to a relatively high rate of decomposition of. the titanium halide and to the production of a relatively large mass of pure titanium metal. Operation of the apparatus is therefore not limited to any critical amount of titanium deposition and thereby makes lpossible relatively large-scalepro'- duction of pure titaniummetal. If desired, the amount of titanium metal produced within a Snglevesselmay be increased by providing there within a plurality of the previously described titaniumdeposition shellsfeach heated by its ownj heating element positioned therewithin as well as: by radiant heatl from the' neighboring shell or shells.

It will also beA noted that the creation of substantiallyidentical vacuum conditions within both the titanium-deposition shell and the decomposition chamber relieves the shell structure from mechanical stresses. due. to a difference in pressure. on opposite sides or" the shell walls. This feature is of. particular advantage inasmuch as it. permits` the use., in theY construction of the titaniumfdeposition shell, of graphite which is relatively weak in the structural sense -butwhich is Iparticularly advantageousias a titanium-deposition surface because of the ease withA which the deposited metal can be separatedtherefrom. Moreover, where the shell isconstructed of the same metal as that being deposited, the shell maybe made of relatively thin gauge stockY which will not be subjected to serious'mechanical stress. In the latter case, the deposited metalforms a continuation of the mass of the shell and need not be separa-ted therefrom. 1n the preferred form of .our apparatus, as shown in Fig. l,V the provision of afsmally opening permitting gaseous communication .between the interior of the shell and the'decompositionchamber at all times not only eliminates a pressure differential between the'interior of the Shell and the chamber during the evacuation stage ybut also, during the halide decompositionY stage while, at the Sametime, insuring the substantial exclusionfrom the heating element atmosphere of decomposable titanium halide although such an atmosphere prevails in communicationV therewithwithin the halide decomposition chamber.

Although the apparatusy and itsoperation have been described hereinbefore solely inl connection withrtheproduction of metallic'titanium, it will be' clearly app-arent that the apparatus is equally well adapted to the production of other. similar metals such as zirconium and hafnium, and mixtures thereof, which can-,be produced advantageously under similar conditions. by high temperature decompositionn of their respective halides.

lWe claim: l. Apparatus for producing, a metal by thermal decomposition of the metal halide which comprises a closed vessel provided with hermetically sealed walls and adapted to beA evacuated, the vessel being provided with at least one openable wall in order to permit removal of a metal-deposition shell positioned therewithin, a metaldeposition shell removably positioned withinsaid vessel in such manner` as to dene a substantially unobstructed chamber between the shell and the walls of said vessel, an electrical .heatingelement positioned within the shell and'adapted to heat the shell essentially by thermal radiation toa temperature sufficient: to eiect thermal decom-` positionof the metal halide incontact therewith, means for evacuating the shelland1the chamber; and a vapor inlet and a vapor outlet-.communicating withthe chamber for establishinga moving metal halideatmosphere Within the chamber in contact with the metal-deposition shell but substantially out of Contact with the heating element.

2. Apparatus for producing a metal by thermal decomposition. of the. metalv halide which comprises a closed vessel provided with* hermetically sealed walls and adapted to be evacuated, the vessel being provided with at least one openable wall in order to permit removal. of. avmetal-deposition shell positioned therewithin, a substantially closed metal-deposition shell removably positioned within said vessel in such manner as. to deiine ay substantially unobstructed chamber between the shell and the walls ofsaid vessel, thel metal-deposition shell being provided with a` small opening providing gaseouscommunication between the shelland the chamber, an electrical heating element positioned within the shell and. adapted to heat the shell essentially by thermal' radiation to aA temperature. sufficient tov effect thermal decomposition of the metal halide. in contact therewith, means for evacuating.- the shell and the chamber, and a vapor inlet and a vapor outlet communicating with the chamber for establishing a moving metal halide atmos. phere within the chamber in Contact with the metal-deposition shell but substantially outof contact with the heating element.

3. Apparatus for producing a metal by thermal decomposition of the metal halide which comprises a closed vessel provided with hermetically sealed walls and adaptedY to be evacuated, the vessel being providedl with at least one openable wall in order to permit removal of a metal deposition shell positioned therewithin, a metaldeposition shell removably positioned within said vessel in such manner as to denne a substantially unobstructed chamber between the shell and the walls of said vessel, an electrical heating element comprising a spiral graphite resistor positioned within the shell and adapted to heat the shell essentially by thermal radiation to a temperature sufcient to. eiect thermal decomposition of the metal halide in contact therewith, rneans for evacuating they shell and the chamber, and a vapor. inlet and a vapor outlet communicating with the chamber for establishing a moving metal halide atmosphere within the chamber in contact with the metal-deposition shell but substantially out ofv contact with the heating element.

4. Apparatus for producing a metal-by thermal decomposition of the metal halide which corn-i prises a closed vessel provided with hermetically sealed walls and adapted to be evacuated, the vessel being provided with at least one openable wall in order to permit removal of a metaldeposition shell positioned therewithin, a metal deposition shell removably positionedk within said vessel in such manner as to denne a closed and substantially unobstructedchamber between the shell and the walls of said vessel, an electricall heating element positioned within. the shell and adapted to heat the shell essentially by thermal radiation to a temperature sufficient to eiect thermal decomposition of the metal halide in contact therewith, means for establishing and maintaining a non-oxidizingr atmosphere in the shell containingV the heating element, means for evacuating the chamber, and avapor inletand a vapor outlet communicating with the chamber for establishing a moving metal halide atmosphere within the chamber in contact with the metal-deposition shell but out of contact with the heating element.

5. Apparatus for producing a metal by thermal decomposition of the metal halide-whichcomprises a: closed vessel provided with hermetically sealed walls and adapted to be evacuated, the vesselv beingprovided with atl least onel openable 9 Wall in order to permit removal of a metaldeposition shell positioned therewithin, an electrically-conductive metal-deposition shell removably positioned within said vessel in such manner as to dene a substantially unobstructed chamber between the shell and the Walls of said vessel, an electrical heating element positioned within the shell and adapted to heat the shell essentially by thermal radiation to a temperature sucient to elect thermal decomposition of the metal halide in contact therewith, electrical contact means interconnecting one portion of the heating element and the shell within the vessel, an electric terminal positioned exteriorly of the vessel insulated from both the shell and the vessel and providing electrical contact with another portion of the heating element, another electric terminal positioned exteriorly of the vessel and providing electrical contact with said shell, whereby the electrical heating element circuit includes the heating element, the contact means and the shell, means for evacuating the shell and the chamber, and a vapor inlet and a vapor outlet communicating with the chamber 10 for establishing a moving metal halide atmosphere within the chamber in contact with the metal-deposition shell but substantially out of contact with the heating element.

6. Apparatus according to claim 5 wherein the shell in the absence of a deposit of the metal thereon is provided with an electrical resistance which comprises an appreciable portion of the total electrical resistance of the heating element circuit.

CHARLES L. SCHEER. CHARLES M. MCFARLAND.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

